1. Field of the Invention
The present invention relates to a semiconductor light emitting apparatus using a semiconductor light emitting element which has an active layer made of an InGaN material and stripe structure, and in which laser oscillation does not occur.
2. Description of the Related Art
Conventionally, various optical-scanning color image printing apparatuses have been provided. Such apparatuses scan a sheet of color sensitive material with three color recording light beams so as to expose the sheet of color sensitive material to the three color recording light beams, and record a color image in the sheet of color sensitive material. These optical-scanning color image printing apparatuses can be broadly classified into two types. In the first type, the three color recording light beams are red, green, and blue light beams, and the usual color sensitive materials for exposure with visible light are used. In the second type, the above three color recording light beams are not red, green, and blue light beams, e.g., the three color recording light beams are in the wavelength range of red to infrared. In the second type of optical-scanning color image printing apparatuses, the three color recording light beams are respectively modulated based on image information on red, green, and blue components of the image, and three photosensitive layers having sensitivities in the wavelength ranges of the three color recording light beams are exposed to the three modulated color recording light beams, respectively.
The above first type of optical-scanning color image printing apparatuses have an advantage that the recording cost is low since the usual color sensitive materials are inexpensive and have stable characteristics. However, these type of optical-scanning color image printing apparatuses require light sources which can emit red, green, and blue laser beams, respectively. For downsizing and weight reduction, it is advantageous to use semiconductor laser devices as the light sources, rather than gas laser devices. Nevertheless, semiconductor laser devices emitting green and blue laser beams are not practically available at present.
In the above situation, Japanese Unexamined Patent Publication, No. 11-74559, which is assigned to the assignee of the present patent application, discloses a semiconductor light emitting element which can emit blue or green light, and realizes small spot emission and a narrow beam divergence angle. The semiconductor light emitting element disclosed in JPP-11-74559 is a so-called superluminescent diode in which laser oscillation does not occur, although the disclosed semiconductor light emitting element has an active layer made of an InGaN material and stripe structure. Since the light emission area in the semiconductor light emitting element is confined in the stripe structure, the disclosed semiconductor light emitting element can emit green or blue light with a small beam diameter and a narrow beam divergence angle.
However, the conventional semiconductor light emitting devices have a drawback that the wavelength of emitted light is prone to vary due to characteristics of the InGaN material, as explained below.
When the relative composition of Indium in the InGaN active layer is increased, the energy gap is reduced, and accordingly the wavelength of light emitted from the InGaN active layer shifts to a longer wavelength side. Therefore, in order to realize emission of green or blue light, the relative composition of Indium in the InGaN active layer must be increased to a certain amount. However, the spatial nonuniformity of the Indium composition in the InGaN active layer increases with increase in the average relative composition of Indium. For example, when the active layer is excited by current injection, carriers move to low energy areas in which the Indium composition is relatively high, and energy relaxation and radiative recombination occur in the low energy areas. Nevertheless, since light emission occurs in the local area in this case, the volume contributing to the light emission is limited. Therefore, the more highly the InGaN active layer is excited, the more carriers flood into areas in which the Indium composition is relatively low. Consequently, the wavelength of the light emitted from the InGaN active layer shifts to a shorter wavelength side.
The above variation in the wavelength of the light emitted from the InGaN active layer is seriously disadvantageous to the aforementioned optical-scanning color image printing apparatuses. Since the color sensitive materials including silver halide photosensitive materials have spectral sensitivities, effective amounts of exposure vary as the wavelength of recording light varies, even if the intensity of the recording light does not vary.
As explained above, in the optical-scanning color image printing apparatuses, suppression of the wavelength variation in the recording light is important as well as control of the intensity of the recording light. In particular, the suppression of variations in the wavelength and intensity of the recording light is important in the color image printing apparatuses used in medical applications in which highly accurate gradation control is required.
An object of the present invention is to provide a semiconductor light emitting apparatus using a semiconductor light emitting element which includes stripe structure and an active layer made of an InGaN material, and emits light without laser oscillation, wherein variation in the wavelength of the light emitted from the semiconductor light emitting element can be effectively suppressed.
Another object of the present invention is to provide a semiconductor light emitting apparatus using a semiconductor light emitting element which includes stripe structure and an active layer made of an InGaN material, and emits light without laser oscillation, wherein variations in the wavelength and intensity of the light emitted from the semiconductor light emitting element can be effectively suppressed.
(1) According to the first aspect of the present invention, there is provided a semiconductor light emitting apparatus comprising: a semiconductor light emitting element which includes a stripe structure and an active layer made of an InGaN material, and emits first light without laser oscillation; an optical resonator combined in with said semiconductor light emitting element; and a wavelength selection means which allows resonance, in the optical resonator, of second light having a selected wavelength included in the first light emitted by the semiconductor light emitting element.
Since the optical resonator is combined with the semiconductor light emitting element, and the wavelength of the light which resonates in the optical resonator is locked to the selected wavelength by the provision of the wavelength selection means, the variation of the wavelength of the light output from the semiconductor light emitting apparatus can be effectively suppressed.
In addition, the light emission area of the semiconductor light emitting element is confined in the stripe structure, so the semiconductor light emitting element can emit green or blue light with a small beam diameter and divergence angle.
Preferably, the semiconductor light emitting apparatus according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (vi).
(i) The optical resonator may be an external optical resonator containing the semiconductor light emitting element.
(ii) In the case (i), the wavelength selection means may be a band-pass filter provided in the external optical resonator.
(iii) In the case (i), the wavelength selection means may be a grating provided in the external optical resonator.
(iv) In the case (i), the wavelength selection means may be a fiber Bragg grating provided in the external optical resonator.
(v) In the case (i), the wavelength selection means may be a Bragg reflection mirror provided in the external optical resonator.
(vi) The optical resonator may be an internal optical resonator provided inside the semiconductor light emitting element.
(2) According to the second aspect of the present invention, there is provided a semiconductor light emitting apparatus comprising: a semiconductor light emitting element which includes a stripe structure and an active layer made of an InGaN material, and emits first light without laser oscillation; a wavelength selection means which is arranged in an optical path of the first light emitted by the semiconductor light emitting element, and allows output, from the semiconductor light emitting apparatus, of only second light having a selected wavelength included in the first light emitted by the semiconductor light emitting element; an optical detector which detects intensity of the second light, and outputs a detected result; and an output regularizing unit which drives the semiconductor light emitting element based on the detected result so as to regularize the intensity of the second light.
The regularization of the intensity of the second light means that the intensity of the second light detected by the optical detector corresponding to an arbitrary level (or value) of a driving signal is maintained constant, i.e., the intensity of the light output from the semiconductor light emitting apparatus corresponding to an arbitrary level (or value) of a driving signal is maintained constant, where the driving signal is supplied from the outside of the semiconductor light emitting apparatus for manipulating the intensity of the optical output of the semiconductor light emitting apparatus.
Since the wavelength selection means is provided in the optical path of the first light emitted by the semiconductor light emitting element, only the second light having a selected wavelength included in the first light emitted by the semiconductor light emitting element is output from the semiconductor light emitting apparatus. Therefore, the variation in the wavelength of the light output from the semiconductor light emitting apparatus can be suppressed.
However, in the case where only the wavelength selection means is provided, the intensity of the second light which is allowed to be output from the semiconductor light emitting apparatus varies when the spectrum of the first light emitted from the semiconductor light emitting element varies. Therefore, such a semiconductor light emitting apparatus cannot be used in the aforementioned optical-scanning color image printing apparatuses due to the variation of the intensity.
Nevertheless, since the optical detector and the output regularizing unit are provided in the semiconductor light emitting apparatus according to the second aspect of the present invention, the output regularizing unit drives the semiconductor light emitting element, based on the intensity of the second light detected by the optical detector, so as to control the intensity of the second light to be maintained constant for an arbitrary level of the driving signal. That is, the variation of the intensity of the light output from the semiconductor light emitting apparatus can also be suppressed. Therefore, the light output from the semiconductor light emitting apparatus according to the second aspect of the present invention can be used in applications in which light having a stabilized wavelength and intensity is required.
Preferably, the semiconductor light emitting apparatus according to the second aspect of the present invention may also have one or any possible combination of the following additional features (vii) to (xii).
(vii) The output regularizing unit may comprise an automatic power control circuit.
(viii) The wavelength selection means may be a band-pass filter formed on an optical output end surface of the semiconductor light emitting element.
(ix) In the case (viii), the band-pass filter may be made of an optical multilayer thin film.
(x) In the case (viii), the band-pass filter may include an optical absorption material.
(xi) In the cases (viii) to (x), the optical output end surface may be inclined at a certain angle away from a plane perpendicular to a direction in which the stripe structure is arranged, and the certain angle may be equal to or greater than one degree.
(xii) The wavelength selection means may be a band-pass filter arranged separately from the semiconductor light emitting element.
The semiconductor light emitting apparatuses according to the first and second aspects of the present invention can realize a light source which can emit light in the visible short wavelength range including the green and blue wavelengths with a small beam diameter and divergence angle, and the wavelength of the emitted light does not vary. Therefore, the semiconductor light emitting apparatuses according to the first and second aspects of the present invention can be used in recording (printing or reproducing) a color image by using light of the three primary colors, i.e., red, green, and blue, exposure of high gradation silver halide photographs, laser display apparatuses, and the like.
Further, the semiconductor light emitting apparatuses according to the first and second aspects of the present invention can replace the conventional light sources in various fields in which laser is used. A typical example of such a field is fluorometric analysis, where a gas laser is conventionally used as an excitation light source.